Mechanical vibrating system



June 16, 1936.

R. A. HEISING MECHANICAL VIBRATING SYSTEM Fil ed March 27, 1955 PIEZOELECTRIC OR OTHER INSULATOR T 22 I ll/1 INVENTOR RAHEIS NG AT TOR/VEVPatented June 16, 1936 UNITED STATES PATENT OFFICE MECHANICAL VIBRATINGSYSTEM Raymond A. Heising, Summit, N. J., assignor to Bell TelephoneLaboratories,

Incorporated,

10 Claims.

This invention relates to a mechanical vibrating system and particularlyto means for efficiently driving an elastic mechanical vibratory elementby a piezoelectric crystal element.

A piezoelectric crystal driven mechanical vibratory element isalternative to tuning forks, bars, or the like, as conventionally drivenby electromagnetic or electrostatic means, in fact, has certainparticular advantages thereover which commend it for use in certainfields. However, the prior means of coupling the driving crystal elementto the driven vibratory element, commonly involving the use of acement-like material or a frictional engagement, have not been all thatmight be desired as not being mechanically secure for a sufficientlylong life, as being unadaptable to variation of coupling, and as tendingto introduce anomalous frequency effects due to the use of masses ofmaterial not directly concerned with the driving or vibratory functions.

It is therefore an object of the invention to provide a positivecoupling between the piezoelectric driving element and the mechanicalvibratory element, a coupling which will tend to be mechanically securethrough all exigencies of use, which is not attended by the use ofextraneous material, and which is adaptable to variation so as toprovide, not only a satisfactory form of, and desired closeness of,coupling between the two significant elements, but also a coupling whichitself has a frequency desirably outside the range of frequency of themechanical vibratory element.

The vibration system of the invention comprises a pair of elasticmechanical vibratory ele ments or, as alternately considered, a dividedelastic mechanical element, between which, or the portions of which, apiezoelectric crystal element is compressibly engaged by means of a nutor turnbuckle threaded on the adjacent ends of the two mechanicalvibratory elements, or by means of bolts passing through flanges orcollars on the adjacent ends of the elements. Variation in the closenessof coupling may be accomplished in the one case by narrowing the threadsnear the ends of the mechanical vibratory elements next adjacent to thecrystal element as compared with the more remote threads, so thatcontact be- 50 tween the mechanical vibratory elements and the nut orturnbuckle occurs only on the threads at a desired distance from thepiezoelectric crystal; or, in the other case, by varying the dimensions,and thereby the stiffness, of the flanges or collars, and/or by varyingthe length, diameter and number of the bolts engaging the flanges orcollars.

In either form of the coupling employed, the single piezoelectriccrystal element contemplated above may be replaced by two similarcrystal 5 elements of the so-called perpendicular cut and facedoppositely so as to add their individual effects in phase.

Although ideally the ultimate frequency is that characteristic of themechanical vibratory ele ment alone, it is to be presumed that thisfrequency is somewhat affected by the mass of the nut or turnbuckle orother coupling means, so that the frequency is actually a function, comopositely, of the mechanical vibratory element proper and the couplingmeans. Hence, it may be desirable to: make the mass of the couplingmeans as small as possible and its material to simulate as nearly aspossible that of the vibratory element proper.

The nature and purpose of the invention will be more clearly understoodfrom the following detailed description and by reference to theaccompanying drawing, of which: 7

Fig. 1 illustrates one form of the invention, with means for varying thecloseness of coupling;

Figs. 2 and 3 show modified forms of the invention; and,

Figs. 4 and 5 are circuit diagrams illustrating uses and applications ofa piezoelectric crystal driven mechanical vibratory element utilizingthe couplingv means or methods of the invention.

In all of the figures of the drawing similar reference characters areused to designate sim-' ilar parts.

Referring to Fig. 1, in which some details of the invention are shownsomewhat out of scale for the sake of clearness, the numeral Idesignates a metal rod divided into halves and threaded on the adjacentends between which a plate or disc 2 of piezoelectric material is heldunder compression by means of a nut 3 engaging the threads of the twohalves of the rod. Adjacent to one face of the crystal element is a thinmetal plate or disc 4 intended to serve as an electrode, to which isconnected a lead 5 passing through an orifice 6 in the nut and insulatedtherefrom by suitable means. The electrode 4 is insulated from the rodby means of a plate or disc I of some rigid insulating material such asglass, quartz or porcelain. The rod itself, being in contact with theother face of the crystal element, serves as a second electrode and tosome convenient point thereon is electrically connected a lead 8, which,with the lead 5, may be used to connect the device in an electricalcircuit.

As most suitable for the object in view the piezoelectric plate (crystalelement) 2 is preferably a quartz plate of the so-called perpendicularcut, utilizing longitudinal vibrations in the thickness direction; thatis to say, the plate is cut with its faces having such relation to theprincipal axes of the natural crystal that when an alternating voltageis applied to the electrodes there will be compressional vibrations ofthe plate perpendicular to its principal faces, which may be transmittedto the rod and impart to it a longitudinal vibration. This requires thatthe element be cut from the natural crystal so that its principal facesare perpendicular to certain sides or facets of the natural crystal.

The rod I may be of any metal of good elastic properties, as, forexample, steel. If it is composed of a suitable alloy having a lowtemperature coefiicient of expansion, this will be advantageous inmaintaining a constant frequency and a rod of such material is thereforeto be preferred.

The nut 3 is preferably of the same material as the rod and should be ofsuch mass and dimensions that its natural frequency is much greater thanthat of the rod alone, so as not to complicate the function primarily inmind, that is, the driving of the rod at its own frequency by thecrystal whose natural frequency is different.

The mechanism of operation of the device is as follows: With the nut 3adjusted so that the crystal is under compression, an alternatingvoltage of high frequency applied to the exciting electrodes by means ofthe leads 5 and 8 will cause the crystal to expand and contract alongits thickness direction in synchronism with the applied voltage, andthese forced vibrations will be transmitted to the rod I by virtue ofits elasticity and that of the coupling nut 3. When the frequency of thealternating voltage is of the right value, the rod and nut will be setinto vigorous resonant vibration at the natural frequency of thecombination, which is the desired result. Under this condition there isa counter electromotive force developed by the crystal on the twoelectrodes due to its alternate expansion and contraction, which may bemade use of in electrical circuits in exactly the same manner as in thecase of a vibrating crystal alone.

Since it is in general desirable to have such a degree of couplingbetween the crystal and the vibrating rod as to set up vibrations of thegreatest possible amplitude, it is advantageous to be able to adjustthis coupling. One method by which this may be accomplished is to narrowsome of the threads on the ends of the rods adjacent to the crystal,such as 8a and 9 so that no pressure is exerted thereon by the. nut whenin compression adjustment. The pressure is then taken up by threadsfurther apart, as by threads I0 and those beyond, so that a greaterlength of material of the nut exists between the areas of application ofthe pressure. Under this condition, for the same force exerted by thecrystal in its expansion it will be able to expand further and hence agreater amplitude of vibration can be imparted to the rod. The shiftingin this manner of the areas of pressure for the most desirable degree ofcoupling can best be determined by experiment. In this connection it isobvious, of course, that the thickness of the nut will also be adetermining factor in the coupling.

An essential feature of the invention is that the piezoelectric crystalis at all times held under compression between the two halves of themechanical vibratory element. To accomplish this object satisfactorilyvarious modifications of the form of coupling above described may beemployed, one such modification being shown in Fig. 2. In thisillustration the two halves of the rod I, instead of being threaded asin Fig. 1, are provided with flanges II on the adjacent ends, integralwith the material of the rod, and through holes I2 in these flangesbolts I3 of the same or similar material are passed and tightened bynuts I4 so that the piezoelectric plate 2 may be compressibly engaged asbefore.

Another modification of the form of coupling is illustrated in Fig. 3,wherein collars I5 of suitable material, preferably the same as the rod,are shown slipped over the rod in contact with flanges I I such as areprovided in Fig. 2, the two halves of the rod being drawn together bymeans of bolts I3 and nuts I l engaging the collars, in the same manneras in the case of the flanges I I in the preceding example.

In each form of the coupling illustrated, the elastic properties, massand dimensions of the material comprising the coupling means, as alreadyindicated with reference to Fig. 1, should be such that the vibrationsof the piezoelectric plate are most effectively transmitted to the rod.To this end it is essential that the natural frequency of each componentpart of the coupling,

or the natural frequency of the coupling as a whole, shall be greaterthan that of the rod. In the modifications shown in Figs. 2 and 3, thecloseness of coupling will depend upon the length, diameter and numberof the bolts I3, and the stiffness, and hence the dimensions, of theflanges I i and, in the case of Fig. 3, upon the dimensions of thecollars I5, and hence the coupling may be adjusted for best results byaltering one or more of these factors. In so doing, however, due regardmust be had for the effect of the mass of the coupling means upon thenatural frequency of the composite ensemble.

As a further modification of the invention, the insulating plate I maybe replaced by a second piezoelectric crystal cut in the perpendiculardirection like the plate 2 but faced in the opposite direction. When thepiezoelectric elements are so faced oppositely, they tend to contract orexpand in like phase and therefore the amplitude of crystal movement maybe doubled. In this arrangement the metal plate 4 becomes one excitingelectrode for both piezoelectric plates, the other effective electrodebeing the rod itself.

As an illustration of how a crystal driven mechanical resonator coupledin the manner of this invention may be used in place of an ordinarypiezoelectric crystal alone, reference may be had to Fig. 4, which showsa simple and Well-known type of vacuum tube oscillator circuit. In thisfigure, I is the crystal driven rod, I6 is a threeelement vacuum tubewith filament battery I1 and plate battery I8, I9 is an inductance inthe plate circuit paralleled by a condenser 20, and 2I is a highresistance connecting the grid directly with the filament. The couplingbetween the plate and grid circuits is the capacity existing between theplate and grid electrodes of the vacuum tube, and the condenser 20serves to tune the plate circuit to approximately the same frequency asthe mechanical frequency of the crystal driven rod in order tofacilitate the transfer of energy from the plate circuit to the gridcircuit. The oscillation frequency is determined by the naturalfrequency of the rod.

As another illustration of its application, there is shown in Fig. acrystal driven resonator used as a circuit element in an electricalfilter. In this illustration the filter is a recurrent ladder typestructure, of which two sections are shown, each section containing acrystal driven resonator I in series with the line and a suitablecondenser 22 in shunt with the line. It is well known that properlydesigned filters of this character are well suited to pass a narrow bandof frequencies with very sharp out-ofis.

What is claimed is:

l. A piezoelectric crystal driven vibrating system comprising apiezoelectric crystal driving element, an elastic non-piezoelectricmechanical vibratory driven element, positive variable coupling meansbetween said elements, and means for utilizing the characteristicelastic vibratory frequency of said driven element.

2. A piezoelectric crystal driven vibrating system comprising incombination, a piezoelectric crystal driving element, a dividednon-piezoelectric mechanical vibratory driven element, meanscompressively engaging said crystal element between the portions of saiddivided mechanical vibratory element, and means for utilizing thecharacteristic elastic vibrational frequency of said driven element.

3. A mechanical vibratory system comprising a divided mechanicalvibratory element of generally rod-like form and including constrainingmeans at adjacent ends of the component portions thereof, apiezoelectric crystal device between said ends, and a coupling meanscoacting with said constraining means for compressively engaging saidcrystal element between the portions of said mechanical vibratoryelement.

4. A mechanical vibratory system like that of claim 3 in which thecoupling means is so dimensioned that its natural frequency of vibrationis substantially greater than that of the mechanical vibratory element.

5. A mechanical vibratory system like that specified in claim 3 in whichthe adjacent ends of the vibratory element portions are screw threadedto constitute the constraining means and the coupling means is a nut orturnbuckle element threadedly engaged with the ends of the vibratoryelement and turned up so as to compressively engage the crystal devicetherebetween.

6. A mechanical vibratory system like that specified in claim 3 in whichthe adjacent ends of the vibratory element portions are flanged toconstitute the constraining means and the coupling means comprisesclamping means engaging the relatively remote flange surfaces with suchconstraint as to compressively engage the crystal de- 5 vice betweensaid vibratory element portions.

7. A mechanical vibratory system like that specified in claim 3 in whichthe adjacent ends of the vibratory element portions are flanged toconstitute the constraining means and the coupling means comprises oneor more sets of bolts and nuts ooacting, through orifices in saidflanges, with the relatively remote surfaces of the flanges tocompressively engage the crystaldevice between the vibratory elementportions.

8. A mechanical vibratory system like that specified in claim 3 in whichthe adjacent ends of the vibratory element portions are flanged toconstitute the constraining means and the coupling means comprises apair of collars each individual to a flange and assembled on a vibratoryelement portion to engage the flange surface most remote from thepiezoelectric crystal device together with clamping means coacting withsaid collars so as to oompressively engage the crystal element betweenthe vibratory element portions.

9. A mechanical vibratory system like that specified in claim 3 in whichthe adjacent ends of the vibratory element portions are screw threadedto constitute the constraining means and the coupling means is a nut orturnbuckle element threadedly engaged with the ends of the vibratoryelement and turned up so as to compressively engage the crystal devicetherebetween,

a certain consecutive number of threads on the vibratory elementportions next adjacent to the crystal device being relatively deformedso that no initial pressure is exerted thereon by the nut when incompression adjustment, whereby a predetermined length and mass of thenut material may be caused to exist between the areas of application ofthe pressure to correspondingly permit an optimum degree of expansion ofthe crystal device and hence a maximum amplitude of vibration can beimparted thereby to the mechanical vibratory element.

10. A mechanical vibratory system like that specified in claim 3 inwhich the piezoelectric crystal device comprises a pair of piezoelectriccrystal elements with an electrode element therebetween.

RAYMOND A. HEISING.

